Vehicle brake systems as well as other hydraulic systems require brake or hydraulic fluid which is free of entrained air bubbles and water vapor. Both of these substances, if present in the system, deteriorate the quality of the fluid by lowering the boiling point and increasing the compressibility of the fluid. This condition causes spongy operation thereby preventing the desired quick and firm response to an input force. Atmospheric air is the source of humidity and water as well as air bubbles. The fluid, particularly brake fluid is hygroscopic, thus tending to absorb moisture from the ambient air. To prevent air and water from entering the system during manufacture it is customary to take special care in filling the system. In particular, it is known to evacuate the system to remove air and water prior to filling with the fluid.
The U.S. Pat. No. 3,339,401 to Peters shows a brake fill machine which has an overhead mounted portable coupling device coacting with a vehicle chassis on a production line. A vacuum source evacuates the brake system, a brake fluid supply fills the system, a flow responsive unit and a vacuum sensor check for leaks, and a fluid salvage device receives excess fluid. According to the disclosure suitable valving is operated by an electrical control to cause the brake lines to be evacuated, a vacuum sensor tests for leaks and, if the brake lines are acceptable, valves are opened to fill the brake lines under pressure. Other U.S. patents disclosing the method and apparatus for evacuating a brake or other hydraulic system and then filling with fluid are Larson U.S. Pat. No. 4,017,329, Wimbush U.S. Pat. No. 4,503,678, Wimbush U.S. Pat. No. 4,506,507 and Janashak et al U.S. Pat. No. 4,708,175.
In order to successfully carry out the scheme of evacuating a brake system and filling it with fluid it is necessary to manage the fluid supply to avoid contamination of the fluid and especially to protect it from contact with atmospheric air. It is also important to prevent the formation of air bubbles in the fluid when pumping or otherwise transferring the fluid. In the event the fluid, as received, contains air or moisture it is important to remove those contaminants. To accomplish these goals it is the practice when filling automotive brake systems to maintain a vacuum over the fluid at each stage of fluid handling. It has been found that less extreme and more reliable techniques can be used in place of much of such vacuum systems.